drm/i915: Refresh VLV/CHV PSR comments on HW PSR_state machine.

DK had pointed out a comment there was hard to understand, so I
tried to read back again and I couldn't understand that as well.
So let me re-phrase that in a way that anyone can understand
later, even myself.

Also fixed the comment block style.

v2: Accept DK's suggestion on PSR_state 2 and PSR_state 3 named
    as spec.

Cc: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Dhinakaran Pandiyan <dhinakaran.pandiyan@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20170912183059.5086-1-rodrigo.vivi@intel.com